I don't know how much time I will have when I will be at Mutianyu. So I have a question.

For an average hiker, the northern route from no.14 to no.23 takes 2 - 3 hours for a one-way trip.
If you start from no.10, it may need an extra hour. Hope you enjoy your staying there!

Sorry my bad English. I mean another thing.
I have seen the map on the top of this page.
I try to repeat (sorry):
if I'm on tower 10,
and I want to return down to "cable car ticket office&north entrance" using the "north climbing route".
How long is this piece of road? (I mean how much time by walk from tower10 to the north entrance)
I can use this if I haven't time to go until tower 6

Oh, you mean going downhill from the tower 10?
Generally speaking, it will spend 30 to 40 mins to arrive at the North Entrance on that tourist map. :)